Electrochemical oxidation route of methyl paraben on a boron-doped diamond anode

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Electrochimica Acta 117 (2014) 127– 133

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Electrochimica Acta

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Electrochemical oxidation route of methyl paraben on a boron-dopeddiamond anode

Juliana R. Steter, Robson S. Rocha, Dawany Dionísio, Marcos R.V. Lanza, Artur J. Motheo ∗

Universidade de São Paulo–Instituto de Química de São Carlos Avenida do Trabalhador Sancarlense 400, Centro, CP 780 São Carlos - SP,CEP 13560-970, Brazil

a r t i c l e i n f o

Article history:Received 21 August 2013Received in revised form18 November 2013Accepted 19 November 2013Available online 4 December 2013

Keywords:Endocrine disruptorElectrochemical oxidationBoron-doped diamond anodeMineralization route

a b s t r a c t

Parabens have been widely used in different industries and can be found in health and personal careproducts. They are esters of p-hydroxy-benzoic acid associated with breast tumors and classified asendocrine disruptors. This study describes the galvanostatic electrochemical oxidation of methyl paraben(MePa) on a boron-doped diamond anode using current densities in the 1.35 to 21.6 mA cm−2 range. Thedegradation process can be controlled by either charge transfer or mass transport, according to the exper-imental conditions and rate of mineralization of MePa increased by the current density. The concentrationvariation as a function of electrolysis time showed that the degradation kinetics follows a pseudo first-order law. A mechanism for the MePa degradation based on reactive intermediates determined by gaschromatography mass spectrometry (GC-MS) is also proposed.

© 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, probably no issue concerning environmentaltoxicology has drawn more attention from the scientific com-munity and regulatory authorities than endocrine disruptors.According to the U.S. Environmental Protection Agency (EPA),an endocrine disruptor is any exogenous substance that inter-feres with the synthesis, secretion, transport, conjugate activity,metabolism or elimination of the natural hormones responsible forthe homeostasis regulation, reproduction, development and behav-ior of living species [1–4].

Endocrine disruptors can be classified as persistent, lipophylicand bio-accumulative and are low-vapor-pressure compounds,characteristics that facilitate their accumulation in the environ-ment. These substances are often found in industrial effluentsand/or domestic sewage and are only partially removed by theconventional sewage treatment [5–10].

Several natural and synthetic substances have been classifiedas endocrine disruptors: phenolic compounds, pesticides, pharma-ceutical and therapeutic agents, natural estrogens, phytoestrogens,phthalates, parabens, heavy metals, among others [2].

Parabens are a very common class of preservatives widely usedin food and pharmaceutical industries and especially in health andpersonal care products (HPC: health and personal care). Chemically,

∗ Corresponding author. Tel.: +55 16 3373 9932; fax: +55 16 3373 9952.E-mail address: [email protected] (A.J. Motheo).

they are designated as esters derived from p-hydroxy-benzoic acidand the ones most commonly used are methyl paraben, which ismore hydrophilic, and butyl paraben, the most soluble. Their solu-bility properties make them convenient to use [11–13]. The humanpopulation is exposed to parabens from different sources becausethese substances have been used over last 50 years in several indus-tries. They can be found in most of the cosmetic and food productsand are used in over 13,200 formulations [12]. Products contain-ing methyl paraben may have direct contact with the skin, lips,hair, nasal and oral mucosal, axillas and their use may be over aperiod of years. By this via, these industrial products are releasedin the environment on a daily basis, causing problems as bioaccu-mulation and affecting the human health. These esters have beenfound in breast tumors, probably associated to the use of deodor-ants, lotions or sprays, and several studies have classified them asendocrine disruptors [14–19].

Research and development efforts, as well as the implemen-tation of processes and methods for a more efficient treatmentof sewage and wastewater have been conducted [20–22]. Theseefforts include the development of methods for either the removalor degradation of those endocrine disruptors taking into accountthe efficiency and cost of those processes for the implementa-tion of such technologies. Advanced oxidation processes (AOP)including chemical, photocatalytic, electrochemical and ozone oxi-dations, sonolysis, Fenton, photo-Fenton and other techniqueswhich involve the in situ generation of the hydroxyl radical (•OH)have been considered effective technologies in the degradation ofa wide variety of organic compounds [23–42].

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One of the most promising techniques for the oxidation oforganic contaminants is the electrochemical process, which offersseveral advantages, such as versatility, environmental compatibil-ity and easy automation [43–47]. Several anodes have been used forthe electrochemical treatment of wastewaters, e.g., dimensionallystable anode (DSA®), SnO2-Sb, �-PbO2 and boron-doped diamond(BDD). However, in recent studies, the use of BDD has shown that,because of the electro-generation of larger quantities of (•OH),strongest oxidant species–Eq. (1), the electrooxidation process isfaster and more efficient [48–53].

H2O → •OHads + H+ + e− (1)

This study deals with the electrochemical oxidation of MePausing a BDD as anode and the analysis of parameters, such ascurrent density applied, speed and degradation kinetics, mineral-ization current and energy efficiencies. Based on the determinationof the degradation products, as well as aromatic and aliphaticintermediates, a mechanistic route for the degradation of MePa isproposed.

2. Experimental

2.1. Materials

Methyl paraben from Aldrich (reagent grade), K2SO4 fromMallinckrodt, K3[Fe(CN)6], K4[Fe(CN)6] and Na2CO3 from Synthwere used as received. Acetonitrile and methanol were chro-matografically pure (J. T. Baker). All solutions were prepared withultrapure water (>18 M�, Milli-Q, Millipore Inc.).

2.2. Electrochemical setup and conditions

MePa (100 mg L−1 in 0.05 mol L−1 K2SO4 aqueous solution) wasoxidized in a one-compartment pyrex cell (400 mL) operated at25 ± 1 ◦C in batch mode. A BDD thin film deposited on Nb withsingle crystal p-type Si wafer (substrate containing 500 ppm ofboron), provided by Adamant Technologies (La-Chaux-de-Founds,Switzerland) was used as anode and its effective working (geo-metric) area was 9.68 cm−2. A titanium foil of approximately samearea of the anode was used as cathode and an Ag/AgCl electrodewas used as reference. The range of the current densities used was1.35 to 21.6 mA cm−2 on a galvanostat/potenciostat from Autolab(model PGSTAT30 N).

2.3. Procedures and analysis

The concentrations of MePa in the solution were determined byHPLC (Shimadzu LC-10AD VP) with a Zorbax SB-C18 column and adetector set at �= 254 nm, corresponding to the maximum absorp-tion wavelength of MePa. Samples of 20 �L were injected and amixture (40:60 (V/V) acetonitrile:water) flowing at 1.0 mL min−1

was used as the mobile phase. The retention time corresponding tothe MePa was 5.3 minutes.

The percentage of mineralization was monitored by theabatement of the total organic carbon (TOC) determined by a TOC-VCPH (Shimadzu). UV-Vis absorption spectra were recorded on aMultiSpec-1501-Shimadzu by a temperature accessory model CPS-Controller set at � = 254 nm.

The current efficiencies (CE) were calculated as

CE =2.67

[TOC0 − TOCf

]FV

8It× 100 (2)

where (TOC0) and (TOCf) correspond to the TOC values (g L−1)at initial and final times, respectively, F is the Faraday constant(96,487 C mol−1), V is the volume of the working solution (L), I is

-1 0 1 2

-2

-1

0

1

2

3

(4)

(3)(2)

(1)

j / m

A c

m-2

E vs. (Ag/AgCl) / V

(a)

Fig. 1. Cyclic voltammograms of BDD anode in: (a) 0.05 mol L−1 K2SO4 (blank solu-tion) and (1 to 4 cycles) 100 mg L−1 methyl paraben in 0.05 mol L−1 K2SO4. Cyclicvoltammograms performed in the potential range -1.6 to +2.0 V vs. Ag/AgCl andscan rate of 50 m Vs−1.

the current intensity (A), t is the time interval (s) and 2.67 is theconversion factor of the COD (Chemical Oxygen Demand) to TOC[28].

The energy consumption EC (kW h m−3 and kW h Kg−1) definedas the electrochemical oxidation energy for the removal of 1 g ofparaben can be calculated as follows [54,56] considering U thepotential cell (V), I the current applied (A), V the solution volume(m3), m the converted mass (kg) and t the time (h):

EC = UIt

1, 000(V) or (m)(3)

The degradation products of MePa were identified by GC-MSon a Varian CP-3800 Gas Chromatograph equipped with an iontrap Saturn 2200 Mass Spectrometer using Agilent DB -5 capillarycolumn (30 m x 0.25 mm, 0.25 �m film thickness). Methodologyapplied: 70 ◦C hold (1 min), rate of 15 ◦C min−1 up to 170 ◦C, hold(2 min), rate of 35 ◦C min−1 up to 250 ◦C. The injector temperaturewas kept at 80 ◦C and helium (1 mL min−1) was used as the carriergas. The mass ranged from 40 to 250 m/z. Before injections in GC-MS, the products of the sample solutions degradation were filtratedand extracted six times with methanol by solid phase extraction(SPE) using C18 filters.

3. Results and discussion

3.1. Cyclic voltammetry

The BDD electrode was characterized by cyclic voltammetry.Initially, the electrode was subjected to an electrochemical con-ditioning, i.e., anodic polarization for 30 minutes using a solutionof K2SO4 (0.05 mol L−1) at constant current of 50 mA cm−2, for theremoval of any deposits or impurities from the electrode surface.Fig. 1 shows the cyclic voltammograms of the BDD electrode inMePa 100 mg L−1 in 0.05 mol L−1 K2SO4 aqueous solution, poten-tial range of -1.6 to +2.0 V vs. Ag/AgCl electrode and scan rate of50 mV s−1.

There are two oxidation peaks in the potential range applied,one located at +0.75 V and another at +1.25 V. The first anodicpeak (+0.75 V) can be assigned to the hydroxyl group present inthe molecule, which is easily oxidized based on the peak dis-appearance after four potential scans. The second peak (+1.25 V)corresponds to the carboxyl group, which is more difficult to be oxi-dized than the hydroxyl group, because the electron donor effectof the carboxyl group deactivates the aromatic ring and prevents

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its oxidation. After four voltammetric cycles, the current decreasesand both peaks disappear perhaps as a result of the formation of apolymeric film partially blocking the electrode surface, and conse-quently, inhibiting the oxidation processes. However, BDD anodeshave inert surfaces with low adsorption capacity and high activ-ity for the oxygen evolution reaction, which makes these surfacesextremely unfavorable to the formation of polymeric films and tosuch films to stay on the surface [57–59]. Thus, in the present casewhat may be happening is the partial deactivation of the originalsurface by forming a polymer film not very adherent, which can beeasily removed by treatment at high values of anodic potential and,consequently reactivating the surface.

3.2. Current density effect

The relationship between the limiting current density andchemical oxygen demand (COD(t)) can be expressed by

jlim = 4FkmCOD(t) (4)

where jlim(t) is the limiting current density (A m−2) at time t, F is theFaraday constant (96,487 C mol−1), km is the mass transfer coef-ficient in the electrolytic cell (m s−1) and COD(t) is the chemicaloxygen demand (mol m−3), which can be converted to TOC values[55].

Depending on the current applied during the electrolysis, twodistinct processes can be identified [55]:

(i) jappl < jlim: the reaction kinetics is controlled by charge transfer,i.e. the organic species are oxidized on the electrode surface,and the current efficiency (CE) is around 100%.

(ii) jappl > jlim: the reaction kinetics is controlled by mass transportand secondary reactions, as oxygen evolution, resulting in adecrease in the current efficiency and CE < 100%.

CE is an important parameter to characterize the electrochemi-cal process. It is defined as the fraction of the current applied to thedegradation of one or more substances present in aqueous solu-tions.

The mass transport coefficient (km) value under hydrodynamicconditions was determined by current diffusion techniques usingsolutions of 0.1 mol L−1 K3[Fe(CN)6], 0.05 mol L−1 K4[Fe(CN)6] and0.5 mol L−1 Na2CO3. The potential difference between the cath-ode and the anode was gradually increased and the correspondingcurrent values were recorded until a current plateau had beenobtained. These values are proportional to the concentration of ironions in the solution and called limiting current of the redox process.The relationship between km and jlim and the total concentration ofiron in the solution are given by

km = jlimnFA[Fe2+/Fe3+]

(5)

where n is the number of the electrons involved in the oxidation ofspecies Fe(CN)6

4− and Fe(CN)63− and A is the anode area (m2).

Considering the mass transport coefficient (km) equal to2.57 × 10−5 m s−1, the cell used and the working electrode, the cur-rent limit was determined as 5.44 mA cm−2. Thus, to perform theelectrochemical degradation of MePa, the values of current densitywere selected below and above the limiting current value.Oxidationprocedures

The galvanostatic electrolyses were carried out at constant cur-rent density values in the 1.35 to 21.6 mA cm−2 range for thedetermination of the operational system processes (charge transferor mass transport) and their corresponding efficiencies.

Fig. 2 shows the dependence of current efficiency on the currentdensities applied. These values are much lower when operating inthe charge transfer regime, because the total charge supplied is

0 5 10 15 20

25

50

75

100

Cu

rren

t ef

fici

ency

japp / mA cm-2

japp < jlim

japp > jlim

Fig. 2. Current efficiency (CE) as a function of the applied current density (japp)for the 120 min. electrolysis of 100 mg L−1 methyl paraben in a BDD anode from0.05 mol L−1 K2SO4 solution at 25 ◦C.

used in the MePa degradation process. For high current density val-ues the mass transport process predominates and part of the chargeis converted to parallel reactions, as oxygen evolution, therefore theefficiency of the system decreases considerably.

The energy consumption (EC) estimated by Eq. 3 increases whenthe current applied increases (Table 1), in accordance with theresults reported by Costa et al. However, discrepancies betweenour results and those from the literature correspond to differencesin the experimental setup, i.e. electrochemical configuration.

Fig. 3A and 3B show the dependence of TOC on the electrol-ysis time and the specific charge applied, respectively. Both plotsindicate the partial MePa degradation considering the TOC removalvalues increase in function of the current density and reach 37.76and 47.08% for current densities of 10.8 and 21.6 mA cm−2, respec-tively, after 120 minutes. The behavior observed in Fig. 3 can beassociated with the increase in the •OH generation according tothe current density or specific charge applied [59].

As previously mentioned, the oxidation experiments were per-formed considering current density values above and below thelimit current. For the current densities of 1.35 and 2.70 mA cm−2,which are below jlim, the process is ruled by a charge transfer andthe removal is inefficient. A significant improvement in the effi-ciency is observed for current densities above jlim, therefore theprocess is ruled by mass transport and the solution is saturatedwith many species, such as oxygen, peroxyl and hydroxyl radical[60,61].

3.3. Kinetic Analysis

The degradation of MePa follows pseudo first-order kinetics(Fig. 4), with apparent constant rate (kapp/10−3 min−1) dependenton the current density applied. According to Table 1, the removalof higher percentages, considering concentration decay valuesfrom HPLC, were obtained for current densities of 2.70 mA cm−2

(34.65%) and 5.44 mA cm−2 (31.61%), which provided constantrate values of 3.59 and 3.12 × 10−3 min−1, respectively. On theother hand, considering japp = 21.6 mA cm−2, the rate constant is1.86 × 10−3 min−1, less than 68% in comparison with that deter-mined for japp = 1.35 mA cm−2 (kapp = 2.74 × 10−3 min−1), thereforethe charge transfer process predominates.

The kinetic results can be explained according to the mechanismof degradation of methyl paraben, which may occur by three dis-tinct forms: i) direct oxidation at the electrode surface; ii) indirect

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Table 1Energy consumption, kinetic parameters, concentration decay percentage values and �TOC removal for the anodic oxidations of the 100 mg L−1 methyl paraben in0.05 mol L−1 K2SO4 using a BDD anode at 25 ◦C, considering 120 minutes of electrolyses, applying current densities of 1.35, 2.70, 5.44, 10.8 and 21.6 mA cm−2.

japp/mA cm−2 U/V EC/kW h m−3 EC/kW h Kg−1 kapp(10−3)/min−1 TOC0-TOCf/% c0-ct/%

1.35 4.06 0.26 1.49 2.74 14.74 30.122.70 4.21 0.54 3.28 3.59 22.82 34.655.44 (jlim) 4.60 1.21 7.01 3.12 35.42 31.6110.8 5.26 2.75 15.61 2.93 37.76 30.0721.6 6.93 7.24 36.0 1.86 47.08 19.85

0 200 400 600 800 100030

40

50

60(B)(A)

TO

C / m

g L

-1

Q / mA h L-1

0 20 40 60 80 100 120

40

50

60

TO

C /

mg

L-1

time / min

Fig. 3. Effect of the applied current density on TOC removal as a function of (A) electrolysis time and (B) consumed specific charge for the degradation of 100 mg L−1 of methylparaben in 0.05 mol L−1 K2SO4 aqueous solution, using a BDD anode at 25 ◦C during 120 minutes. (�) 1.35, (©) 2.70, (�) 5.44, (�) 10.8 and (�) 21.6 mA cm−2.

oxidation mediated by species such as S2O82-; iii) indirect oxidation

mediated by hydroxyl radicals.The initial proposal of the present work was to minimize the

secondary reactions and to maximize the performance of the pro-cess and so, the current densities were adjusted to values aroundthe limiting current calculated. It means that the experiments wereperformed in an intermediated condition i.e., between the two pro-cesses controlling the methyl paraben degradation: charge transferand mass transport. Additionally, for higher current density values(10.8 and 21.6 mA cm−2) the process efficiency decreases becausethe increased influence of the side reactions.

0 40 80 120

0,0

-0,1

-0,2

-0,3

-0,4

ln c

t / c 0

time / min

Fig. 4. Methyl paraben concentration decay during 120 min. electrolysis of100 mg L−1 methyl paraben in a BDD anode from 0.05 mol L−1 K2SO4 solution at25 ◦C by applying current densities of (�) 1.35, (�) 2.70, (�) 5.44, (�) 10.8 and (©)21.6 mA cm−2. Kinetic analysis performed by assuming a pseudo-first-order methylparaben degradation.

The efficiency of the methyl paraben degradation presented isnot just related to the greater amount of hydroxyl radicals in thereaction medium since that the recombination of these radicals canoccurs. This recombination will favor the formation of H2O2 thatcan subsequently be oxidized to molecular O2. Thus, the degrada-tion efficiency can be properly explained by the reactivity of theintermediate species formed during the process.

In the proposed mechanism of the methyl paraben degradation,the limiting step of the process is the removal of the p-substitutedgroups of the aromatic ring. Thus, the limiting step is the degra-dation of III and IV tautomers presented in Fig. 6. A characteristicof phenolic compounds is that their oxidation is favored by directelectron transfer occurring at the electrode surface (or in the caseof BDD in the vicinity of the electrode surface). Thus, according tothese results, the highest values of kapp are those values obtainedat low current densities, for which the direct oxidation is favored,and consequently, the direct degradation of intermediates gener-ated in the limiting step of the process is favored. It is importantto observe that the results obtained for 2.7, 5.44 and 10.8 and mAcm−2, respectively, 3.6, 3.1 and 2.9 × 10−3 min−1, are in accordancewith the experimental error.

The value of kapp determined at 21.6 mA cm−2

(1.86 × 10−3 min−1) is lower, as expected, since under theseconditions the degradation occurs with the interaction of methylparaben and its intermediates with hydroxyl radicals, favoring theformation of species such as H2O2 and O2 molecules and not thelimiting step of the process.

The acidic species, including the aromatic acid such as p-hydroxy-benzoic acid, and aliphatic acids as malic, tartaric andoxalic, formed from the breakdown of the aromatic ring, interactwith hydroxyl radicals. These interactions increase the importanceof the hydroxyl radicals, and so, the higher the concentration ofthese radicals in solution, the greater will be the efficiency of theprocess. By GC-MS analyses, such compounds start to predominate

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after 300 min of electrolysis and so, it can be concluded that thekinetic analysis based on experiments of 120 min shows that thedegradation is faster at low current densities.

3.4. Efficiency of MePa mineralization

Complete mineralization is understood as the entire conversionof an organic molecule to CO2. In the case of MePa, the reaction thatrepresents this process is described by:

C8H8O3 + 13H2O → 8CO2 + 34H+ + 34e− (6)

where 34 electrons are involved in the complete degradation of1 mol of MePa.

The mineralization efficiency (ME) can be calculated as [62]:

ME = �TOCexp

�TOCtheor× 100 (7)

where �TOCexp is the organic carbon removal at a particular time tand �TOCtheor is the theoretical reduction of organic carbon consid-ering the electrical charge applied and consumed in the overallprocess of MePa mineralization.

Fig. 5 shows that at the beginning all the current applied isused to decompose MePa and the mineralization efficiency is themaximum. Over time, MePa is gradually converted into its interme-diates and this oxidation decreases the ME. An increase in the TOCremoval implies an ME decrease, because smaller amounts of MePaare transported to the BDD surface to react with (•OH). This MePareduction on the surface favors side reactions, as evolution oxygenreaction, confirmed by the behavior of the curve corresponding to21.6 mA cm−2 in Fig. 5.

Mineralization efficiency was calculated by the equation 7 onlyto facilitate the selection of a value of current density to be used

0 10 0 20 0

75

80

85

90

95

100

limjjapp ≥

(A)

Min

eral

izat

ion e

ffic

iency

(%

)

Q / mA h L-1

(B)

limjjapp ≤

0 30 0 60 0 90 0

Fig. 5. Mineralization efficiency (estimated by Equation 7) for the electrochemicaldegradation of 100 mg L−1 methyl paraben in a BDD anode from 0.05 mol L−1 K2SO4

solution during 120 min. at 25 ◦C as a function of the consumed specific charge.Anodic oxidations performed at (�) 1.35, (©) 2.70, (�) 5.44, (�) 10.8 and (�)21.6 mA cm−2.

in the exhaustive electrolysis, however, the discussion about theefficiency of the electrochemical process was based on EC valuesdetermined by equation 2.

The overall mineralization was obtained (92%) for the exhaus-tive experiment (300 minutes at 10.8 mAcm−2), while for theexperiments of 120 minutes, the maximum removal TOC obtainedwas around 50% to 21.6 mA cm−2. Can be considered that high cur-rent densities applied as 10.8 and 21.6 mA cm−2 are adequate for

OCH3

O

HO

OH

methyl paraben

OH

O

HO (I)HO

(-COOH)

(II)

OH

HO

OH O

O(III) (IV)

OH

OH

OH

O

O

(V)

OH

OH

OH

OH

O

O

HO

(VI)

OH

OH

O

O

(VIII)

OH

OH

OH

OH

O

O

HO

(IX)

OH(-COOH)

(-COOH)

HO

OH O

OH

O OH

(VII)

(-COOH)

O

OH

O

HO

(X)

CO2 + H2O

Fig. 6. Proposed mechanism for the electrochemical degradation of 100 mg L−1 methyl paraben in a BDD anode from 0.05 mol L−1 K2SO4 solution applying 10.8 mA cm−2 for300 min at 25 ◦C. The intermediates of the reaction were identified by GC/MS/MS.(I) m/z = 136, (II) m/z = 94, (III) m/z = 110, (IV) m/z = 108, (V) m/z = 116, (VI) m/z = 133, (VII) m/z = 149, (VIII) m/z = 104, (IX) m/z = 119, (X) m/z = 90.

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the mineralization process, indicating that there are not consider-able amounts of organic compounds in the final solution. However,there is a decrease in the CE values for the oxidation of the methylparaben.

3.5. Route proposed for the degradation of MePa

During the electrolysis of 300 min at 10.8 mA cm−2, sampleswere collected at 60 min intervals to identify the aromatic andaliphatic substances formed during the process by GC technique.The analysis of the detected reaction intermediates enables thedetermination of a reactional sequence for the total degradationof the MePa (Fig. 6). Initially, there is a nucleophilic addition of thehydroxyl radical to the carbon of the carbonyl group (C�+) presentin the MePa molecule, followed by the elimination of methoxyl(-OCH3) as a “living group” to form p-hydroxy benzoic acid (I).

This intermediate (I) underwent two consecutive processes:decarboxylation (-COOH) followed by hydroxylation (•OH inser-tion). Phenol (II) is formed as a reactive intermediate in thedecarboxylation process. When the hydroxylation process occurs,the ortho and para positions of the aromatic ring are active for thesubstitution reactions, specially the p position, because of the elec-tron donor effect, then the catechol intermediate (III) and/or itstautomer benzoquinone (IV) can be formed.

In this step of the mechanism there is a rupture in the aromaticring and aliphatic compounds can be identified. Compound (V) hasa double bond and can undergo (•OH) insertion, in such a way thattwo new intermediaries, i.e. VI (malic acid) and VII (tartaric acid)can be formed.

Intermediate VI can undergo hydroxylation followed by decar-boxylation generating compound VIII (malonic acid), whose C�+is susceptible to attack by the hydroxyl radical, generating com-pound IX, the 3-hydroxy-propanedioic acid. In this stage, both VIIand IX may be converted to oxalic acid (X) and either degraded ormineralized forming CO2 and H2O [63].

4. Conclusions

The total mineralization of 100 mg L−1 MePa in0.05 mol L−1 K2SO4 aqueous solution was achieved after 300-minute galvanostatic electrolysis at 10.8 mA cm−2 using a BDDanode. The analyses of different parameters in 120-minute electrol-ysis allow concluding that the TOC removal increases in function ofthe current density applied as a consequence of an excessive con-centration of hydroxyl radical species in the solution, also favoringparallel reactions, as oxygen evolution and hydrogen peroxidegeneration. The kinetic analysis of the MePa degradation showsthat it follows a pseudo first-order and, considering the removal inmg L−1 and apparent constant rate (kapp/min−1), the best resultsare achieved with current densities of 2.70 and 5.44 mA cm−2,which are values under the major influence of charge transfer.These experimental observations are important to establish theinitial operating conditions of reactors in galvanostatic mode withBDD electrodes, in particular to treat effluents or waste containingparaben in sulfate aqueous medium. Finally, a mechanism for theelectrochemical degradation of methyl paraben is proposed byCG-MS analyses of the aromatic and aliphatic intermediates.

Acknowledgments

The authors would like to acknowledge the Brazilian researchfunding agencies National Council for Scientific and Technolog-ical Development (CNPq), Federal Agency for the Support andImprovement of Higher Education (CAPES) and São Paulo Research

Foundation (FAPESP) for the financial support provided to thisresearch.

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